1) Field of the Invention
The present invention relates to an insulating film forming method and a semiconductor device, and more particularly to a method of forming a multi-layer insulating film structure including an insulating film made of silicon carbide and an insulating film made of insulating material having a dielectric constant lower than that of silicon oxide and to a semiconductor device having this multi-layer structure.
2) Description of the Related Art
In a multi-layer wiring structure of a semiconductor integrated circuit device, a transmission speed of an electric signal depends upon a wiring resistance and a parasitic capacitance between wiring lines. As the integration degree of recent semiconductor integrated circuit devices becomes higher, the distance between wiring lines becomes shorter and the parasitic capacitance becomes larger. In the generation in which the wiring distance is equal to or larger than 1 μm, a delay of a signal transmission speed caused by the parasitic capacitance had only a small influence on the process speed in a whole device. However, at a wiring distance of 0.5 μm or shorter, the influence of the parasitic capacitance between wiring lines became large, and especially at a wiring distance of 0.2 μm or shorter, it can be anticipated that the parasitic capacitance between wiring lines greatly influences the process speed in a device.
The parasitic capacitance between wiring lines can be reduced by thinning wiring lines. However, as the wiring line is thinned, the wiring resistance increases, hindering a high speed operation of a semiconductor integrated circuit device. A low dielectric constant of an insulating film is effective for reducing the parasitic capacitance without an increase in the wiring resistance.
Known low dielectric insulating materials include organic polyallylene and polyallyl ether, inorganic hydrogen silsesquioxane (HSQ), hybrid methyl silsesquioxane (MSQ), and mixture of HSQ and MSQ, respectively used for forming an insulating film by coating. Silicon oxycarbide has drawn attention as low dielectric constant insulating material, which is used for forming an insulating film by chemical vapor deposition (CVD) using organosiloxane-containing material as source gas.
A technique of lowering a dielectric constant is known by which voids are formed intentionally in a low dielectric constant insulating film independently from the film forming process used. This technique is associated with the development of low dielectric constant material and the progress of application to semiconductor integrated circuit devices, while maintaining a balance between a lowered dielectric constant and a lowered mechanical strength of a film caused by forming voids. Porous low dielectric constant material has recently been developed which has a lowered relative dielectric constant of 2.0 to 2.5 while realizing a Young's modulus of 5 to 7 GPa.
While interlayer insulating films for a wiring layer and a via layer of a damascene structure are made to have a lower dielectric constant, a barrier layer for preventing diffusion of conductive material copper (Cu) has also been desired to have a lower dielectric constant. Known good materials of the barrier layer for preventing copper diffusion include silicon nitride (SiN), oxygen-doped silicon carbide and the like, a film made of which is formed by CVD. The relative dielectric constants of silicon nitride and oxygen-doped silicon carbide are about 7.0 and 4.5, respectively, which cannot be said a low dielectric constant.
Attention has been paid recently to oxygen-undoped silicon carbide which is used for forming a film by CVD in a low oxygen density condition or a no-oxygen condition by using organosilane as source gas. An oxygen-undoped silicon carbide film contains only a small amount of oxygen or does not contain oxygen so that the relative dielectric constant can be lowered to about 3.5 or smaller while maintaining a good barrier performance of copper diffusion.
The surface of a copper diffusion barrier layer is generally hydrophobic. If a film made of low dielectric constant insulating material is formed on a hydrophobic surface, the adhesion of the film is likely to become poor. Japanese Patent Laid-open Publication No. 2004-88047 discloses the technique of forming a film made of low dielectric constant material after a hydrophilic process is performed for a film made of ESL3 manufactured by Novellus Systems Japan. In forming an ESL3 film, tetramethylsilane and carbon dioxide are used as source gasses. According to this method, the hydrophilic process is performed by making the surface of an ESL3 film contact with aqueous solution of ammonium fluoride. This hydrophilic process can enhance the adhesion between the silicon carbide film and the low dielectric insulating material film on the silicon carbide film.
A film made of ESL3 manufactured by Novellus Systems Japan contains oxygen. If a low dielectric constant insulating material film is to be formed on an insulating film made of oxygen-undoped silicon carbide, a sufficient adhesion property cannot be obtained by using only the hydrophilic process disclosed in Japanese Patent Laid-open Publication No. 2004-88047.